Competitive uptake of dimethylamine and trimethylamine against ammonia on acidic particles in marine atmospheres

Alkaline gases such as NH3 and amines play important roles in neutralizing acidic particles in the atmosphere. Here, two common gaseous amines (dimethylamine (DMA) and trimethylamine (TMA)), NH3, and their corresponding ions in PM2.5 were measured semicontinuously using an ambient ion monitor-ion chromatography (AIM-IC) system in marine air during a round-trip cruise of approximately 4000 km along the coastline of eastern China. The concentrations of particulate DMA, detected as DMAH+, varied from <4 to 100 ng m–3 and generally decreased with increasing atmospheric NH3 concentrations. Combining observations with thermodynamic equilibrium calculations using the extended aerosol inorganics model (E-AIM) indicated that the competitive uptake of DMA against NH3 on acidic aerosols generally followed thermodynamic equilibria and appeared to be sensitive to DMA/NH3 molar ratios, resulting in molar ratios of DMAH+ to DMA + DMAH+ of 0.31 ± 0.16 (average ± standard deviation) at atmospheric NH3 concentrations over 1.8 μg m–3 (with a corresponding DMA/NH3 ratio of (1.8 ± 1.0) × 10–3), 0.80 ± 0.15 at atmospheric NH3 concentrations below 0.3 μg m–3 (with a corresponding DMA/NH3 ratio of (1.3 ± 0.6) × 10–2), and 0.56 ± 0.19 in the remaining cases. Particulate TMA concentrations, detected as TMAH+, ranged from <2 to 21 ng m–3 and decreased with increasing concentrations of atmospheric NH3. However, TMAH+ was depleted concurrently with the formation of NH4NO3 under low concentrations of atmospheric NH3, contradictory to the calculated increase in the equilibrated concentration of TMAH+ by the E-AIM.